It is an object of this invention to provide a method and apparatus for continuously proving correct operation of such electronic circuits and to ensure failure to safety of electronic circuits and systems in a railway control arrangement whilst avoiding conventional parallel redundancy arrangements.
The invention is particularly useful in systems where a decision concerning safety is based upon the output signal, so that incorrect operation of the signal processing circuits producing that signal can directly affect the safety of the system. For example, a mass transit system, such as an underground railway, having an automatic train protection system in which a coded signal representing safe maximum train speed is transmitted to the train for analysis by the train and comparison with a signal representing the measured speed of the train. If the coded signal is incorrectly decoded or is corrupted before comparison then a wrong decision concerning the maximum speed at which it is safe for the train to travel may be the result.
Also in the railway signalling field, the invention may be useful in coded track circuits in which a coded signal is transmitted between a track circuit transmitter and track circuit receiver via the running rails. When the section of rails, i.e. the track section, is unoccupied the signal reaches the receiver, but when a train is present in the section the rails are electrically shorted together thus preventing propagation of the signal along the section. The present invention may be applied to the receiver apparatus in order to verify its correct operation at all times. A railway track circuit receiver of this type to which the invention may be applied is described in U.S. Pat. No. 4,516,249, assigned to the same assignee as this application.